The invention relates to methods and apparatus for producing an enhanced raster image.
Raster images use a grid of small picture elements (pixels) to represent graphics. Raster images may contain one or more channels that represent information about the color elements in the image. In the red, green, blue (RGB) color model with a depth of 8 bits, for example, there are three channels, each of which can take on a value of 0 to 255 so that any one of over 16 million different colors can be assigned to any pixel in the raster image. A raster imaging system typically produces a visual representation of a raster image by scanning successive lines of pixels onto a surface. Common raster imaging systems include laser xerographic, inkjet, electrostatic, thermal transfer, magnetographic, dot matrix, ion deposition, laser film, and laser erosion systems. The arrangement of pixels on a printed page or on a CRT screen gives a viewer the illusion that a continuous image is being observed. The degree to which the pixel arrangement can simulate an ideal image depends upon a number of characteristics of the imaging system, including spatial addressability, pixel size, dynamic density range of pixels (number of gray or color levels), placement consistency, and consistency of the rendering process.
Sometimes an image will be distorted as a result of an image-capturing process. For example, some regions of a photographic image may be in focus (sharp) and other regions may be out of focus (blurry). An image is generally in focus when features of the image are defined by edges with sharp intensity transitions; an image is out of focus when the edge regions are not characterized by sharp intensity transitions. One way to reduce this kind of distortion is to increase the relative intensities of the pixels located at the edges of features in the image. For example, as described in Adobe.RTM. Photoshop.RTM. 4.0 User's Guide, published by Adobe Systems Incorporated (1996), the Adobe.RTM. Photoshop.RTM. imaging processing software, version 4.0, includes an Unsharp Mask filter that enables a user to increase the relative intensity values of edge pixels by a user-selected factor (specified as a percentage of the relative intensity between adjacent pixels). A user can also specify a relative intensity threshold (with a value from 0 to 255), whereby only adjacent pixels with relative intensities that are above the threshold will be modified. The Unsharp Mask filter locates every two adjacent pixels with a difference in intensity values that is greater than the threshold, and then increases the relative intensity between these pixels by the user-specified factor. The user can also specify the number of surrounding pixels to which the sharpening effect will be applied. The Unsharp Mask filter applies the same sharpening factor to each pixel with a relative intensity value above the threshold.
The Adobe.RTM. Photoshop.RTM. imaging processing software, version 4.0, also includes a Blur filter and a Custom filter, as well as a number of other image filters. The Blur filter smooths transitions by averaging the pixels where significant color transitions occur in an image. The Custom filter enables a user to reassign a given pixel's intensity value based upon the intensity values of surrounding pixels. The Custom filter allows a user to select the factors by which to multiply the intensity values of a target pixel based on the values of the pixels immediately adjacent to the target pixel. The user then selects a scale factor by which to divide the sum of weighted pixel intensity values and an offset value to be added to the result of the scale operation. Once defined, the Custom filter can be applied to each pixel in the image.